A completed photovoltaic cell which is well known in the art includes a layer of cadmium sulfide (CdS) disposed on a suitable substrate, and a second material which forms a heterojunction or "barrier layer", in cooperation with the CdS. The material typically used to form a heterojunction with CdS is cuprous sulfide, Cu.sub.x S, x being less than 2 where non-stoichometric cuprous sulfide is formed over the CdS. The technology to mass produce photovoltaic cells which incorporate the Cds--Cu.sub.x S heterojunction is rapidly developing and is not, per se, a subject of the instant invention.
To provide for large scale terrestrial application, the individual photovoltaic cells must be formed into an interconnected array covering large areas. Typically, CdS-Cu.sub.x S heterojunctions produce a voltage of 0.4-0.5 volts. If a higher output voltage is desired in order to transmit or use directly the output power from the photovoltaic cell array, the individual cells may be connected in a series arrangement to produce output voltages of 12-24 volts, i.e., output voltages which are equivalent to voltages produced by present day storage batteries.
Early attempts to provide photovoltaic arrays generally consisted of taking individual photovoltaic cells, adhering those cells to a common substrate, and then interconnecting the photovoltaic cells with wire conductors to complete the array. Such photovoltaic arrays were custom fabricated and were expensive to produce. The requirement to provide for a large number of conductor wires further reduced the availability of surface area for active photovoltaic power generation and thereby reduced the overall efficiency of the photovoltaic array.
The availability of polycrystalline CdS as a component in a photovoltaic cell has greatly increased the capability of forming a series connected array of such cells. U.S. Pat. Nos. 3,483,038, to Hui et al, and 3,571,905 and 3,713,893 to Shirland are typical of recent prior art attempts to provide a large area solar cell array. Both the Hui and Shirland patents teach a process for forming the solar cell array wherein the individual cells are formed as discrete units on the substrate. A mask is placed over the area which is not to receive the deposited material, the selected material is vacuum evaporated and the mask is then removed to complete the formation of that particular layer. A new mask is required for each patterned layer which is to be deposited and an alternating sequence of mask application and layer deposition occurs until the completed photovoltaic array has been formed. It may be appreciated that this method is time consuming and is not well adapted to mass production of large scale panel arrays where the cells are series connected.
Yet another technique for producing photovoltaic cells with polycrystalline CdS is to spray suitable solutions onto a substrate where the solution reacts to form a film of the desired material. U.S. Pat. Nos. 3,880,633 and 3,902,920 to Jordan et al, disclose suitable techniques for forming large area photovoltaic cells by the spray method. A glass substrate is moved through a series of spray booths to form layered films of tin oxide, cadmium sulfide, and perhaps cuprous sulfide. It is a feature of these spray processes that each film is formed at a temperature lower than that at which the preceding film is formed. Accordingly, it would be desirable to form the large photovoltaic panel into some number of smaller cells, to be connected in series for increased voltage outputs, only after all of the layers have been formed. Such a technique would minimize the thermal cycling of the glass and the time and energy required to produce the photovoltaic panel.
The disadvantages of the prior art are overcome by the present invention, however, and improved apparatus are provided for forming an array of photovoltaic cells connected in series including a machine for separating a lrge photovoltaic panel into a plurality of photovoltaic cells on a common substrate.